1. Field of the Invention
The present invention relates to a shaft arrangement structure of an engine, and more particularly to a shaft arrangement structure of a semidry sump type engine which has a crank case divided into upper and lower two members, has a transmission room accommodating a transmission within the crank case, and utilizes a lower portion of the transmission room as an oil reservoir chamber of the engine.
2. Description of the Related Art
In a semidry sump type engine mounted on a vehicle such as a motorcycle or the like, a crank shaft is provided within a crank chamber of a crank case, at least an input shaft and an output shaft of a transmission are provided within the transmission room of the crank case, and a change drum and the like of the transmission are provided within the transmission room.
Further, in the case that the crank case is structured such as to be divided into upper and lower two members, the crank shaft, the input shaft and the output shaft are generally arranged in such a manner that axes thereof exist on a mating surface (a split surface) of the upper and lower members of the crank case, while taking an assembling property of each of the shafts, the transmission and the like into consideration (refer to Japanese Patent Publication No. 2-43886).
In the engine mentioned above, in the case of assembling the crank shaft and the transmission within the crank case, before connecting the upper and lower crank case members, the crank shaft, the input shaft and the output shaft positioned on the mating surface are assembled, for example, in the lower member, and the upper member is thereafter connected to the lower member.
However, in the shaft arrangement as mentioned above, since the crank shaft, the input shaft and the output shaft are arranged in a line on the mating surface of the crank case at an interval in a longitudinal direction (front and rear direction) as seen from the side, a dimension in the longitudinal direction of the engine becomes longer, and the engine is enlarged in size.
Further, for example, the input shaft is generally provided with a clutch, particularly a friction clutch for transmitting a rotation of the crank shaft so as to freely engage and disengage, however, in the case that the input shaft is arranged on the mating surface of the crank case as mentioned above, a position of a lower end portion of the clutch becomes lower. Accordingly, an oil capacity (an oil level) of an oil reservoir chamber is limited on the basis of a necessity of avoiding a reduction of an output caused by a stirring or agitation of the oil by the clutch.